Do all family-based green card applicants get a priority date?
No, only the applicants who are eligible under a family-based preference. Immediate relatives of U.S. citizens simply have a ‘receipt date’ for their I-130.

Is it used to determine which applicants go first?
Broadly speaking, yes. An application submitted the 1st of December 2015 will be reviewed before one submitted the 2nd of December 2015.

More specifically, the priority date is used to organize the order of the queue within one green cardcategory (F1, F2A,…). Another parameter of the queue is where the applicant was born: Mexico, the Philippines, all other countries are three of the most common queues within family preferences.

Is it used to determine when applicants will get their green cards?
Yes. The applicant’s priority date is compared to what U.S. immigration agencies call the “final action date”. All applicants with a priority date up to the final action date can have their interview scheduled.